The present invention relates to a driving device which controls a permanent magnet type motor by a sensor-less vector system without using any magnetic pole position sensor.
Heretofore, in a case where a permanent magnet type synchronous motor is controlled without using any magnetic pole position sensor, a vector control is usually used. This vector control is a technique in which a current flowing through the motor is separated into a torque current component and a field current component, and an optimum current value is calculated in accordance with a position of a rotor, so that an efficient control having little torque fluctuation is possible as compared with a rectangular-wave inverter.
To perform such a control, a magnetic pole position of the rotor needs to be grasped, but in a sensor-less vector control for a high speed in which any magnetic pole position sensor is not used, instead of the magnetic pole position sensor, a value of the current flowing through the motor is utilized to estimate the magnetic pole position. To grasp the value of the current flowing through this motor, the current flowing through the motor is detected using a current detector such as a current transformer or a resistance, and the detected current is separated into a field current component Id and a torque current component Iq to estimate the magnetic pole position.
In an actual vector control, with respect to a d-q rotation coordinate system in which the magnetic pole position of the rotor of the motor is a rotary position at a real angle θd, a dc-qc rotation coordinate system is supposed in which an estimated angle θdc is obtained in the control system. An axial error Δθ between the coordinate systems is estimated and calculated. So as to set this axial error Δθ to zero, a voltage command value of the inverter is fed back and corrected, and this allows an actual magnetic pole position to meet a controlled magnetic pole position.
According to such a vector control, it is possible to ideally control a magnitude and a phase of the current for driving the motor by the inverter in accordance with load conditions, and it is possible to realize motor control with high torque, response, performance and precision. On the other hand, there is not any sensor-less vector control system that is usable from a starting state in which any current flowing through the motor cannot be utilized until the high speed is reached. Therefore, there is considered a method and the like in which during the starting, for example, a constant V/F control is performed. In the control, the magnetic pole position does not have to be detected. At a predetermined rotation speed, an initial magnetic pole position set beforehand is used to shift to the vector control (see, e.g., Japanese Patent Application Laid-Open No. 2004-48886).
In addition, in recent years, to realize energy saving, this type of motor has its speed steadily lowered. When the motor is operated at a low rotation speed or when the motor is in a low load state, the whole current (torque current component+field current component) flowing through the motor is reduced. Therefore, the current cannot be detected by resolution of the above-described current detector, and there has been a problem that the detector fails in detecting the magnetic pole position, and the motor runs out of step and stops.
To avoid this problem, it is considered that the resolution (current detecting level) of the current detector is raised, but a problem occurs in the detection of the current at the high speed and with the high load. It has been difficult to use the single detector from the operation at the low rotation speed and with the low load till the operation at the high rotation speed and with the high load.